1. Field of the Invention
The present invention relates to a vacuum station for storing sewage from a vacuum sewage pipe and then delivering the sewage to a sewage treatment plant or the like, and to a method for operating such vacuum station.
2. Description of the Related Art
Heretofore, there has been known a vacuum sewage system which includes a vacuum station having a collection tank and delivers sewage stored in the collection tank to a sewage treatment plant or the like by a pump in the vacuum station. The vacuum station is the equipment in which vacuum serving as a driving force for collecting sewage is created, and the collected sewage is temporarily stored and then transported to a sewage treatment plant, a sewage relay pump station, or a gravity trunk sewer. The vacuum station comprises a vacuum generating apparatus for generating vacuum, a collection tank for temporarily storing collected sewage, a sewage pump for transporting the sewage from the collection tank, and a controller for controlling these equipment.
As a form of vacuum station, there has been a vacuum station in which equipment including a collection tank, a sewage pump, a vacuum pump and the like is provided on the first basement of independently reinforced concrete construction (first story and first basement), and equipment including a controller, a feed tank, a deodorizing device and the like is provided on the first story of the independently reinforced concrete construction. However, this type of vacuum station causes a clogging problem of the sewage pump by foreign matter and a problem of high equipment cost.
On the other hand, in a small-scale vacuum sewage system (for example, expected to be used for about 3 hundred residents), there has been known a unit-type vacuum station which incorporates an ejector in place of a vacuum pump and a sewage circulating pump installed in a manhole because the facility structure is simple and a site for the vacuum station is not required. The ejector-type vacuum station has the advantage of eliminating the need for a vacuum pump, and omitting a sewage pump because a collecting tank without an enclosed structure allows the collected sewage to be discharged therefrom by gravity, thus simplifying the facility structure. However, there is a possibility that the ejector is clogged with foreign matter because an ejector nozzle allows only small-diameter foreign matter to pass therethrough, and the ejector has a low ultimate pressure ranging from −60 kPa to −50 kPa and a low operating efficiency.
Therefore, in a small-scale vacuum station, there has been demanded a vacuum station which is hardly clogged with foreign matter in sucking and discharging sewage, requires a reduced facility cost, and has a good operating efficiency.
For example, in the vacuum sewage system disclosed in Japanese Patent Publication No. 2684526, a single roots-type multistage vacuum pump is used, and normal rotation and reverse rotation of such vacuum pump are automatically controlled, whereby suction of sewage into a collection tank and discharge of the sewage from the collection tank are performed alternately. In this vacuum sewage system, since sewage can be collected or discharged without using a sewage pump, clogging of the system caused by foreign matter hardly occurs, an ultimate pressure is high and operating efficiency is also high because the vacuum pump is employed.
However, in the vacuum sewage system disclosed in Japanese Patent Publication No. 2684526, since only a single collection tank and a single roots-type multistage vacuum pump are provide, if the vacuum pump breaks down, then collection and discharge of sewage cannot be performed. Since the vacuum sewage system is used for the public, it is essential to ensure the safety to prevent the entire system from malfunctioning owing to a breakdown of the vacuum pump, or the like.